1. Field of the Invention
This invention is concerned with analysis of organic additives in plating baths as a means of providing control over the deposit properties.
2. Description of the Related Art
Electroplating baths typically contain organic additives whose concentrations must be closely controlled in the low parts per million range in order to attain the desired deposit properties and morphology. One of the key functions of such additives is to level or brighten the deposit by suppressing the electrodeposition rate at peaks in the substrate surface. Leveling/brightening of the deposit results from faster metal deposition within recessed areas where the additive, which is present at low concentration, is less effectively replenished by diffusion/bath agitation as it is consumed in the electrodeposition process. The most sensitive methods available for detecting leveling and brightening additives in plating baths involve electrochemical measurement of the metal electrodeposition rate under controlled hydrodynamic conditions for which the additive concentration in the vicinity of the electrode surface is well-defined.
Cyclic voltammetric stripping (CVS) analysis [D. Tench and C. Ogden, J. Electrochem. Soc. 125, 194 (1978)] is the most widely used bath additive control method and involves cycling the potential of an inert electrode (e.g., Pt) in the plating bath between fixed potential limits so that metal is alternately plated on and stripped from the electrode surface. Such voltage cycling is designed to establish a steady state for the electrode surface so that reproducible results are obtained. Cyclic pulse voltammetric stripping (CPVS), also called cyclic step voltammetric stripping (CSVS), is a variation of the CVS method that employs discrete changes in voltage during the analysis to condition the electrode so as to improve the measurement precision [D. Tench and J. White, J. Electrochem. Soc. 132, 831 (1985)]. A rotating disk electrode configuration is typically employed for both CVS and CPVS analysis to provide controlled hydrodynamic conditions. Accumulation of organic films or other contaminants on the electrode surface can be avoided by periodically voltage cycling the electrode in the plating solution without organic additives and, if necessary, polishing the electrode using a fine abrasive. The metal deposition rate can be determined from the current or charge passed during metal electrodeposition but it is usually advantageous to measure the charge associated with anodic stripping of the metal from the electrode. The CVS method was first applied to control copper pyrophosphate baths (U.S. Pat. No. 4,132,605 to Tench and Ogden) but has since been adapted for control of a variety of other plating systems, including the acid copper sulfate baths that are widely used by the electronics industry [e.g., R. Haak, C. Ogden and D. Tench, Plating Surf. Fin. 68(4), 52 (1981) and Plating Surf. Fin. 69(3), 62 (1982)].
Acid copper sulfate electroplating baths require a minimum of two types of organic additives to provide deposits with satisfactory properties and good leveling characteristics. The suppressor additive is typically a polymeric organic species, e.g., high molecular weight polyethylene or polypropylene glycol, which adsorbs strongly on the copper cathode surface to form a film that sharply increases the overvoltage for copper deposition. This prevents uncontrolled copper plating that would result in powdery or nodular deposits. An anti-suppressor additive is required to counter the suppressive effect of the suppressor and provide the mass-transport-limited rate differential needed for leveling. Plating bath vendors typically provide additive solutions that may contain additives of more than one type, as well as other organic and inorganic addition agents. The suppressor additive may be comprised of more than one chemical species and generally involves a range of molecular weights.
Both the suppressor and the anti-suppressor additive concentrations in acid copper sulfate baths can be determined by CVS analysis methods based on the effects that these additives exert on the copper electrodeposition rate. For the suppressor analysis, the CVS rate parameter, usually the copper stripping peak area at a given electrode rotation rate (Ar), is first measured in a supporting electrolyte having approximately the same composition as the plating bath to be analyzed but without organic addition agents. Additions of known volume ratios of the plating bath to the supporting electrolyte (or to a background electrolyte having known concentrations of other additives) produce decreases in the CVS rate parameter that reflect the concentration of the suppressor additive. This xe2x80x9cstandard additionxe2x80x9d suppressor analysis is not significantly affected by the presence of the anti-suppressor, which exerts a relatively weak effect on the copper deposition rate at the plating bath dilution levels involved. For the anti-suppressor analysis, a sufficient amount of the suppressor additive, which may be comprised of a plurality of components or species, is added to the supporting electrolyte to produce a background electrolyte exhibiting substantially the maximum suppression of the copper deposition rate (minimum CVS rate parameter). Additions of known volume ratios of the plating bath to this fully-suppressed background electrolyte produce increases in the CVS rate parameter that can be related to the concentration of the anti-suppressor additive. The exact procedures for CVS analysis of acid copper sulfate baths can vary.
Analysis for the suppressor additive (also called the xe2x80x9cpolymerxe2x80x9d, xe2x80x9ccarrierxe2x80x9d, or xe2x80x9cwetterxe2x80x9d, depending on the bath supplier) typically involves generation of a calibration curve by measuring the CVS rate parameter Ar in a supporting or background electrolyte (without organic additives or with known concentrations of interfering additives), termed Ar(0), and after each standard addition of the suppressor additive. For the calibration curve, Ar may be plotted against the suppressor concentration directly, or normalized as Ar/Ar(0) to minimize measurement errors associated with changes in the electrode surface, background bath composition, and temperature. For the suppressor analysis itself, Ar is first measured in the supporting electrolyte and then after each standard addition of a known volume ratio of the plating bath sample to be analyzed. The suppressor concentration may be determined from the Ar or Ar/Ar(0) value for the measurement solution (supporting electrolyte plus a known volume of plating bath sample) by interpolation with respect to the appropriate calibration curve (xe2x80x9cresponse curve analysisxe2x80x9d). Alternatively, the suppressor concentration may be determined by the xe2x80x9cdilution titrationxe2x80x9d method from the volume ratio of plating bath sample (added to the supporting electrolyte) required to decrease Ar or Ar/Ar(0) to a given value, which may be a specific numerical value or a minimum value (substantially maximum suppression) [W. O. Freitag, C. Ogden, D. Tench and J. White, Plating Surf. Fin. 70(10), 55 (1983)]. Note that the effect of the anti-suppressor on the suppressor analysis is typically small but can be taken into account by including in the background electrolyte the amount of anti-suppressor measured or estimated to be present in the plating bath being analyzed.
The concentration of the anti-suppressor additive (also called the xe2x80x9cbrightenerxe2x80x9d, xe2x80x9cacceleratorxe2x80x9d or simply the xe2x80x9cadditivexe2x80x9d, depending on the bath supplier) is typically determined by the linear approximation technique (LAT) or modified linear approximation technique (MLAT) described by R. Gluzman [Proc. 70th Am. Electroplaters Soc. Tech. Conf., Sur/Fin, Indianapolis, Ind. (June 1983)]. The CVS rate parameter, Ar, is first measured in background electrolyte containing no anti-suppressor but with a sufficient amount of suppressor species added to substantially saturate suppression of the copper deposition rate. A known volume ratio of the plating bath sample to be analyzed is then added to this fully-suppressed background electrolyte and Ar is again measured. The Ar measurement is then repeated in this mixed solution after each addition (typically two) of known amounts of the anti-suppressor additive only. The concentration of the anti-suppressor in the plating bath sample is calculated assuming that Ar varies linearly with anti-suppressor concentration, which is verified if the change in Ar produced by standard additions of the same amount of anti-suppressor are equivalent.
Acid copper sulfate baths have functioned well for plating the relatively large surface pads, through-holes and vias found on printed wiring boards (PWB""s) and are currently being adapted for plating fine trenches and vias in dielectric material on semiconductor chips. The electronics industry is transitioning from aluminum to copper as the basic metallization for semiconductor integrated circuits (IC""s) in order to increase device switching speed and enhance electromigration resistance. The leading technology for fabricating copper IC chips is the xe2x80x9cDamascenexe2x80x9d process (see, e.g., P. C. Andricacos, Electrochem. Soc. Interface, Spring 1999, p.32; U.S. Pat. No. 4,789,648 to Chow et al.; U.S. Pat. No. 5,209,817 to Ahmad et al.), which depends on copper electroplating to provide complete filling of the fine features involved. The organic additives in the bath must be closely controlled since they provide the copper deposition rate differential required for bottom-up filling.
As the feature size for the Damascene process has shrunk below 0.2 xcexcm, it has become necessary to utilize a third organic additive in the acid copper bath in order to avoid overplating the trenches and vias. Note that excess copper on Damascene plated wafers is typically removed by chemical mechanical polishing (CMP) but the copper layer must be uniform for the CMP process to be effective. The third additive is called the xe2x80x9clevelerxe2x80x9d (or xe2x80x9cboosterxe2x80x9d, depending on the bath supplier) and is typically an organic compound containing nitrogen or oxygen that also tends to decrease the copper plating rate. In order to attain good bottom up filling and avoid overplating of ultra-fine chip features, the concentrations of all three additives must be accurately analyzed and controlled.
The concentrations of the suppressor and anti-suppressor in acid copper plating baths can be analyzed with good precision in the presence of the leveler additive by the standard CVS methods. At the additive concentrations typically employed, the effect of the suppressor in reducing the copper deposition rate is usually much stronger than that of the leveler so that the concentration of the suppressor can be determined by the usual CVS response curve or dilution titration analysis. Interference from the leveler can be minimized by utilizing a background electrolyte for the suppressor analysis that contains approximately the same leveler concentration as in the plating bath being analyzed, estimated from the bath makeup composition and previous analyses. Likewise, the anti-suppressor concentration can be determined by the LAT or MLAT analysis procedure and the approximate bath concentration of leveler can be added to the fully-suppressed background electrolyte to minimize leveler interference. With some modifications, for example, to account for relatively high leveler activity or to reduce anti-suppressor interference on the suppressor analysis, these CVS procedures provide reliable measures of the suppressor and anti-suppressor additives used in currently-available acid copper electroplating baths. However, a method is needed for measuring the leveler concentration in the presence of interference from both the suppressor and anti-suppressor.
Since the suppressor and anti-suppressor concentrations can be independently determined, the obvious approach based on traditional chemical analysis practice would be to add these interfering additives, at the concentrations measured for the plating bath, to the background electrolyte used for the leveler analysis. In this matrix matching approach, the leveler concentration would be determined from the change in the CVS rate parameter produced by addition of a known volume ratio of the plating bath sample to the background electrolyte. A calibration curve would be generated by measuring the CVS rate parameter as a function of the concentration of leveler added to the same background electrolyte. However, since the suppressor and anti-suppressor concentrations in the plating bath vary with time, calibration curves would be needed for all combinations of suppressor and anti-suppressor concentrations that occur in the plating bath. An average calibration curve could be used but at a significant sacrifice in measurement accuracy. Furthermore, the sensitivity of the analysis to the leveler concentration is poor for some combinations of suppressor and anti-suppressor concentrations encountered during production operation.
There is a critical need for a method of determining the concentration of the leveler additive in acid copper baths with high precision under all bath operating conditions. It is also desirable to avoid the necessity of generating and utilizing a plurality of calibration curves, which is required for CVS analysis based on traditional analytical approaches.
This invention is a voltammetric method for measuring the concentration of a leveler additive in an acid copper sulfate electroplating bath also containing suppressor and anti-suppressor additives. The method is based on measuring the change in copper deposition rate produced by the leveler additive species. Interference from the suppressor and anti-suppressor additives, which also affect the copper deposition rate, is minimized by adjusting their concentrations in a measurement solution (comprised of sample of the plating bath and a background electrolyte), and in the background electrolyte used for calibration, to the optimum levels for analysis of the leveler. The copper deposition rate in this adjusted measurement solution provides an exceptionally sensitive and reproducible measure of the concentration of the leveler additive since the measurement is always made at the optimum concentrations of the interfering suppressor and anti-suppressor additives, which may be substantially higher or lower than their respective concentrations in the plating bath sample. Further improvement is provided by optimizing the voltammetric parameters used to measure the copper deposition rate so as to provide maximum sensitivity to the leveler and minimum interference from the suppressor and anti-suppressor. The optimum suppressor and anti-suppressor concentrations and optimum set of measurement parameters need to be determined for each additive system and, in some cases, for each additive batch. The analysis of the present invention also requires only one calibration curve for a given additive system. The concentrations of the suppressor and anti-suppressor additives in the plating bath sample are usually measured at the time of the leveler analysis but may sometimes be estimated with sufficient accuracy based on previous analyses.
The method of the present invention contrasts sharply with the obvious approach based on conventional analytical practice of utilizing a matrix matched background electrolyte containing the two interfering additives at their concentrations in the plating bath at the time of the analysis. In this conventional case, the change in copper deposition rate in the background electrolyte produced by addition of a known volume ratio of the plating bath sample also provides a measure of the leveler concentration. However, since the suppressor and anti-suppressor concentrations in the plating bath vary with time, calibration curves are needed for all combinations of suppressor and anti-suppressor concentrations that occur in the plating bath during operation. Furthermore, the sensitivity of the analysis to the leveler concentration and the extent of interference from the suppressor and anti-suppressor depend on the relative concentrations of these interfering additives. For some additive systems, additions of the leveler to copper plating solutions containing the normal levels of suppressor and anti-suppressor produces practically no change in the copper deposition rate so that the conventional matrix matching approach cannot be used.
The copper deposition rate for the method of the present invention is preferably determined by cyclic voltammetric stripping (CVS) or cyclic pulse voltammetric stripping (CPVS). The latter is also called cyclic step voltammetric stripping (CSVS). As used in this document, the term xe2x80x9ccyclic voltammetric strippingxe2x80x9d or xe2x80x9cCVSxe2x80x9d implicitly includes the CPVS method, which is a variation of the CVS method. Likewise, the term xe2x80x9cCVS rate parameterxe2x80x9d includes the analogous CPVS voltammetric rate parameters. In these methods, the potential of an inert electrode, e.g., Pt, is cycled in a plating solution, at a constant rate or in steps, so that copper is alternately plated on the electrode surface and anodically stripped back into the solution. Potential cycling improves the reproducibility of the results by establishing steady-state conditions at the electrode surface. A rotating disk electrode configuration is typically used to provide the well-defined hydrodynamic conditions that are also needed for reproducible results. The copper deposition rate is typically measured via the copper stripping peak area with electrode rotation (Ar) but might also be determined from the stripping peak height, or from the electrode impedance, current or charge corresponding to a given cathodic potential or potential range (with or without electrode rotation). Improved reproducibility and accuracy may be provided by using a normalized CVS rate parameter, for example, the ratio Ar/Ar(0) of the stripping peak area for the measurement solution to that for a supporting electrolyte without additives or to that for a background electrolyte with known concentrations of interfering additives.
Preferably, the suppressor and anti-suppressor concentrations are also determined from their effects on the copper deposition rate. In this case, synergy among the various additives is automatically taken into account and an effective or active concentration is obtained, which is more indicative of the performance of the additive in the plating bath than a strictly analytical concentration. The suppressor, which generally exerts a much stronger effect than the other additives at low concentrations, may be determined by CVS response curve or dilution titration analysis. In this case, dilution of the bath sample during standard additions to a background electrolyte renders the suppressor effect dominant. The anti-suppressor may be determined via the CVS linear approximation technique (LAT) or modified linear approximation technique (MLAT) analysis from its effect in increasing the copper deposition rate in a background electrolyte containing sufficient suppressor for maximum copper deposition suppression. Modifications in these analysis procedures may be needed for some additive systems, for example, to account for relatively high leveler activity or to reduce anti-suppressor interference on the suppressor analysis. Generally, the CVS measurement parameters are optimized for each type of analysis (leveler, suppressor and anti-suppressor) and depend on the particular additive system. Such optimization improves measurement sensitivity and reproducibility and helps minimize interference from the other additives. Within the scope of the present invention, the suppressor and anti-suppressor additives may also be analyzed by other methods, e.g., spectrophotometry, electrochemical ac impedance measurements, electrochemical impedance spectroscopy, or high performance liquid chromatography (HPLC).
Another aspect of the claimed invention provides improved precision for the leveler voltammetric analysis by taking into account variations in the inorganic content of the plating bath. Since chloride exerts a strong effect on the functioning of organic additives used in acid copper baths, its concentration should, if necessary, be adjusted to be within the appropriate range (typically, 25 to 100 ppm) in the plating bath sample being analyzed, and in the background and supporting electrolytes used for the analysis. Variations in the chloride, sulfuric acid and copper ion concentrations within the ranges recommended by the bath supplier usually have a negligible effect on the voltammetric analysis results and typically need to be adjusted in measurement solutions only for analyses requiring very high accuracy. If the particular leveler species used exerts a relatively weak effect on the copper deposition rate and/or the leveler concentration in the plating bath is relatively low, however, variations in the copper content of the plating bath can have a significant effect on the results of the leveler analysis. In this case, the accuracy and precision of the leveler analysis of the present invention may be improved by correcting the measurement solution voltammetric rate parameter for the difference in copper ion concentration resulting from addition of the plating bath to the supporting electrolyte.
Further features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings.